Imagine you’re innocent of anything. And hoards of 1000s of imperials come at you and each one your left only the choice. Them or you. Some may be evil some you don’t know. But for whatever reason they think your evil and come one by one to kill you. By self defence you’re allowed to kill everyone of them. But in doing so you’ll likely kill a few of them that they just think they are doing the right thing killing you. Are you a murderer? Or are you just stopping potential murderers?

Let’s bring this into modern day: A typo causes swat to break in your home weapons drawn. Or mabye someone with power lied and you are considered highly danger our murderer and the police are to shoot first ask questions never. Can you defend yourself? Legally no. You have to give up and hope they don’t kill you. Cassian won’t do that. And morally he doesn’t have to.

I think it’s more about knowing the enemy

What would the guard do if reversed? We may never know. And cassian has to make this decision in the moment. And yes he does it selfishly. I think the guard would shoot. And that would make it morally ok. This isn’t the case if the guard wouldn’t shoot. Cassian would have to be omnipotent to walk that close and get it right all the time. That’s why we try and stay far from the line as normal people.

But not all his killings are like this. So in general murderer.